The present invention relates to a light amplifier and a light transmission system using the same, and more specifically to a light amplifier for a wavelength division multiplexing optical signal, so configured to control an amplification level in accordance with the result of the comparison of a predetermined level with the level of a signal partially branched from an amplified optical signal, and a light transmission system using the same.
In a wavelength division multiplexing (abbreviated to xe2x80x9cWDMxe2x80x9d) transmission system using an optical fiber amplifier, it is necessary to stably transmit a wavelength division multiplexing optical signal over a long distance. Generally, each optical fiber amplifier used in the wavelength division multiplexing transmission system is configured to output an optical signal having a constant level for each wavelength.
In the conventional optical fiber amplifier used in the wavelength division multiplexing transmission system, the amplification level of the light amplifier is controlled in such a manner that a portion of the output signal of the light amplifier is branched so that the level of the optical output signal is detected by a photo diode (abbreviated to xe2x80x9cPDxe2x80x9d), and an automatic power control (abbreviated to xe2x80x9cAPCxe2x80x9d) is carried out to maintain the level of the optical output signal at a constant.
The construction of the conventional optical fiber amplifier mentioned above is illustrated in FIG. 4. In FIG. 1, the reference number 1 designates an optical amplifier, and the reference number 2 indicates an optical divider/splitter. The reference number 6 denotes a photo diode, and the reference number 7 shows a comparator. The reference number 8 denotes a reference level generator, and the reference number 13 indicates an exciting light source. In addition, the reference number 10 denotes an optical signal input terminal, and the reference number 11 shows an optical signal output terminal. The reference number 12 designates a wavelength number information input terminal, which receives an actually transmitted wavelength number information through a monitoring and controlling line from a not-shown terminal station. Incidentally, the optical amplifier 1 is an erbium doped fiber amplifier (abbreviated to xe2x80x9cEDFAxe2x80x9d).
In the optical fiber amplifier shown in FIG. 4, the wavelength division multiplexing optical signal inputted to the optical signal input terminal 10 is amplified to a predetermined level by action of the optical amplifier 1, and then, a portion of the optical output signal of the optical amplifier 1 is branched by action of the optical divider/splitter 2, and the level of the optical output signal is detected by the photo diode 6. The comparator 7 compares the detected level with a reference level which is generated in the reference level generator 8 in accordance with the wavelength number of the wavelength division multiplexing optical signal supplied from the wavelength number information input terminal 12. On the basis of the result of this comparison, the level of the exciting light generated by the exciting light source 13 is controlled, with the result that the APC control is carried out to maintain the level of the optical output signal of the optical amplifier 1 at a constant, and therefore, the amplification level of the light amplifier is controlled.
In the conventional optical fiber amplifier, namely, since the level of the optical output signal outputted from the light amplifier is different dependently upon the wavelength number of a multiplexed optical signal, the transmitted wavelength number information is notified to the reference level generator 8, and the detected level of the optical output signal is compared with the reference level corresponding to the wavelength number by action of the comparator 7, so that level of the optical output signal per one wavelength is controlled to be maintained at a constant.
In addition, Japanese Patent Application Pre-examination Publication No. JP-A-10-51388 discloses another conventional light amplification device. This is so configured that a portion of the optical output signal of the light amplifier is branched into two paths, in one of which the level of the branched optical output signal is measured, and in the other of which the level of an optical signal (amplified spontaneous emission, abbreviated to xe2x80x9cASExe2x80x9d) obtained by removing a desired wavelength component from the optical output signal, is measured. Further, the amplification level of the light amplifier is controlled on the basis of a control signal (a signal of the desired wavelength component) obtained by subtracting the signal level of the other branched path from the signal level of the one branched path.
As mentioned above, on the other hand, since the wavelength division multiplexing transmission system using the optical fiber amplifier is required to stably transmit the wavelength division multiplexing optical signal over a long distance. A optical fiber which is a transmission line, involves a loss, and therefore, in order to stably transmit the optical signal over a long distance, it is necessary to compensate for the transmission loss in the optical fiber. For this purpose, a number of low-noise optical fiber amplifiers are located on the way of the optical fiber transmission path. The optical fiber amplifier amplifies the optical signal to a predetermined optical signal level and sends the amplified signal to the optical fiber transmission line. By repeating this operation, the long-distance light transmission is ensured.
However, in the above mentioned conventional optical fiber amplifier of the APC control, since the level of the optical output signal outputted from the optical fiber amplifier is different dependently upon the number of the wavelengths transmitted in a multiplexed manner, the control is carried out to maintain the level of the optical output signal per one wavelength in accordance with the transmitted wavelength number information. Therefore, the level of the ASE light generated in the light amplifier also changes with the change of the level of the optical output signal. On the other hand, since the range of the wavelength detectable to the photo diode for detecting the optical output signal is wide, the photo diode cannot discriminate how much the detected optical signal contains the ASE light. Accordingly, a problem is encountered in that the degree of deterioration of the optical output signal caused by the ASE light greatly changes dependently upon the number of wavelengths transmitted. In other words, the larger the number of the wavelengths transmitted is, the influence of the ASE light to the optical output signal is small, and the smaller the number of the wavelengths transmitted is, the influence of the ASE light to the optical output signal becomes remarkable.
In addition, since the light amplification device disclosed in JP-A-10-51388 mentioned above is targeted for the optical signal of a certain specific wavelength, when a multiplexed optical signal is inputted, it is impossible to measure the level of the ASE light of the other path mentioned above in a high fidelity. Furthermore, since the multiplexed optical signal is outputted without modification in the one path mentioned above, it is impossible to precisely grasp the level of the optical output signal of the light amplifier. As a result, a problem is encountered in that the amplification level of the light amplifier cannot be controlled.
Moreover, in the wavelength division multiplexing transmission system using the conventional optical fiber amplifier, a problem does not occur in the case that a number of wavelengths are transmitted, since the level of the optical output signal is sufficiently larger than the level of the ASE light. However, at the beginning of the introduction of a system, the number of wavelengths transmitted is small and becomes one in a certain case. In such a situation, the level of the optical output signal drops because of the influence of the ASE light generated in the light amplifier. In this case, if a number of optical fiber amplifiers are arranged in a multi-stage structure, a problem will rise in which the ASE light is accumulated so that the optical output signal becomes buried in the ASE light, with the result that the level of the optical output signal drops.
FIG. 3A illustrates the change of the level of the ASE light in comparison with the optical output signal when a single wavelength is transmitted through a number of optical fiber amplifiers arranged in a multi-stage structure. As seen from FIG. 3A, if the number of repeaters is increased from one stage, to two stages, . . . , to (Kxe2x88x921) stages, to K stages (where xe2x80x9cKxe2x80x9d is a natural number), the ASE light is accumulated to have an increased level, with the result that the optical output signal becomes buried in the ASE light and the level of the optical output signal gradually drops.
If the level of the optical output signal drops, the signal-to-noise ratio (abbreviated to xe2x80x9cSNRxe2x80x9d) of the optical signal resultantly deteriorates, resulting in a quality deterioration of the line. Accordingly, in the wavelength division multiplexing transmission system using the conventional optical fiber amplifier, a transmissible minimum wavelength number and a quality-warrantable transmission distance have their limits.
Accordingly, the present invention was made to overcome the above mentioned problems of the prior art.
An object of the present invention is to provide a light amplifier capable of preventing the deterioration of the optical output signal caused by the ASE light at the time of amplifying a multiplexed optical signal, so as to precisely control the amplification level of the light amplifier, and capable of preventing the level drop of the optical output signal even in the case that the number of wavelengths transmitted is one, so that a long-distance transmission having a good line quality can be realized, and a light transmission system using the same.
According to the present invention, there is provided a light amplification device which includes a light amplification means for amplifying a wavelength division multiplexing optical signal to output an amplified optical signal, and a branch means for branching a portion of the amplified optical signal to supply a branch signal, and which is so configured that an amplification level of the light amplification means is controlled on the basis of the result of the comparison of the level of the branch signal with a predetermined level, characterized is that the light amplification device comprises an optical demultiplexing means for separating the branch signal into different wavelength components, a level adjusting means for controlling the level of the different wavelength components independently of each other, and an optical multiplexing means for combining the different wavelength components thus level-controlled, to output a combined signal, so that the amplification level of the light amplification means is controlled on the basis of the result of the comparison of the level of the combined signal with the predetermined level.
In addition, the optical demultiplexing means demultiplexes the branch signal into different paths corresponding to the different wavelength components, respectively, and the paths are provided in the number corresponding to a maximum number in the number of wavelengths transmitted in the wavelength division multiplexing optical signal.
Furthermore, the different wavelength components are level-adjusted so that the wavelength component in the path corresponding to the wavelength transmitted in the wavelength division multiplexing optical signal becomes a maximum level and the wavelength component in the path corresponding to the wavelength other than the wavelength transmitted becomes a minimum level.
In addition, the optical demultiplexing means is constituted of an arrayed waveguide grating, and the optical multiplexing means is constituted of an arrayed waveguide grating.
Moreover, according to the present invention, there is provided a light transmission system for transmitting and repeating a wavelength division multiplexing optical signal by light amplification devices connected through a transmission path in a multi-stage structure, the light transmission system comprising the above mentioned light amplification devices.
An operation of the present invention will be described. A portion of an optical output signal of an optical amplifier is branched, and is further separated by an optical demultiplexor into different paths in accordance with the wavelength. Of the different paths, in the path corresponding to the wavelength transmitted in the wavelength division multiplexing optical signal, the attenuation amount of the optical signal is controlled to a minimum level so that the level of the optical output signal is maintained as it is, and in the path corresponding to the wavelength other than the wavelength transmitted in the wavelength division multiplexing optical signal, the attenuation amount of the optical signal is controlled to a maximum level, so that the level of the ASE light is suppressed or minimized. After the level adjustment, the optical signals of the different paths are combined, and the level of the combined signal is detected. The detected level is compared with a reference level which is determined for each wavelength number, and on the basis of the result of this comparison, the exciting light level of an exciting light source is controlled. Thus, since the amplification level of the optical amplifier is controlled on the basis of the optical output signal having the suppressed or minimized ASE light, the level of the optical output signal is precisely controlled. In addition, if optical fiber amplifiers having this optical amplifier are connected in a multi-stage structure, it is possible to prevent the drop of the level of the optical output signal caused by the ASE light. Therefore, even if the number of the multiplexed wavelengths is only one, a stable long-distance transmission having a good line quality can be realized.